Bipolar batteries are known to the art to consist generally of a series of bipolar plates separated by separators, each bipolar plate comprising positive and negative electrodes on opposite sides of a conductive substrate. It has been attempted to combine several flat bipolar cells in multi-compartment batteries encased in suitable containers. Many of the substrate materials used heretofore are heavy, and it is desirable to minimize the weight of the substrate to reduce overall battery weight. Prior attempts at providing lighter weight substrate materials have resulted in substrates which are deficient in at least some properties desirable for this application. Moreover, it is necessary that substrate materials be able to withstand prolonged exposure to the harsh environment involved with sulfuric acid electrolytes used in most battery systems.
Perhaps the most common type of substrate employed in the prior art is a thin sheet of lead. In a typical prior art bipolar battery, the lead substrate would have a positive electrode coating on one side (for example, porous lead oxide) and negative electrode coating material on the opposite side (for example, sponge lead active material).
It has been suggested that the overall weight of such batteries could be reduced by the use of carbon filled plastic as the substrate, as exemplified in Biddick, et al., U.S. Pat. No. 4,098,967, issued July 4, 1978 for "Electrochemical System Using Conductive Plastic". In this patent, finely divided vitreous carbon is loaded at a rate of 40-80% by volume into a plastic material, with the filled plastic acting as a bipolar substrate. A layer of lead-antimony foil bonded to the carbon-filled plastic provides the surface for adhering the active materials. A lead sheet can be bonded to the substrate to define a low resistance terminal.
Carbon, however, is not entirely stable, especially as a positive electrode material, because of its tendency to oxidize. Thus, bipolar substrates utilizing carbon as a conductive filler are not generally satisfactory for long-term use.
Unitary plate electrodes comprising fiberglass coated with conductive oxides, for example tin dioxide and lead dioxide, and having a thin film of lead or graphite filled resin, are described in Rowlette, et al., U.S. Pat. No. 4,547,443, issued Oct. 15, 1985. This particular use does not, however, suggest that the transition metal conductive oxides could be used in the manner described herein.
The present invention involves the use of conductive oxides, preferably those of titanium, tungsten, vanadium, molybdenum and niobium. Certain oxides of these transition metals exist or can be prepared in a non-conductive state. Reduction of these non-conductive oxides, such as in a hydrogen atmosphere, at elevated temperatures, creates a conductive class of materials whose use in batteries as described herein has not heretofore been recognized.
Certain conductive metal oxides have been used in other applications, for example, in polymeric compositions for electrical components as described in Penneck, et al., U.S. Pat. No. 4,470,898, issued Sept. 11, 1984, and in corrosion-resistant coatings as described in Tada, U.S. Pat. No. 4,352,899, issued Oct. 5, 1982.
Voss, et al., in U.S. Pat. No. 3,096,215, issued July 2, 1963, discloses the use of a sintered titanium dioxide electrode, impregnated with silver, for use as an auxiliary electrode for eliminating gases formed during operation of a battery. The auxiliary electrode is coupled electrically to the positive or negative plates of the battery, depending on which electrode is causing the problem gas generation. Voss, et al. do not suggest the use of any metal oxides in a polymeric binder for use as a substrate material in bipolar electrodes.
Certain metal oxides have also been suggested for use in fuel cells to serve as substitutes for more expensive platinum as a catalyst material. See Nestor, U.S. Pat. No. 3,480,479 issued Nov. 25, 1929 (a molybdenum oxide mixed with tungsten disulfide) and Broyde, U.S. Pat. No. 3,544,378 issued Dec. 1, 1970 (a rare earth tungsten oxide M.sub.x WO.sub.3 where x is between 0 and 1 and M is a rare earth element). These catalyst oriented patents do not suggest the use of such materials in bipolar battery substrates.
An oxygen reducing negative active material for a storage cell which includes a molybdenum oxide having an average valency between 4 and 6 is discussed in Gabano, et al., U.S. Pat. No 3,871,917 issued Mar. 18, 1975. The oxide is supported by mechanical compression. A conductive body (for example, graphite) and binding agents may be employed. The material is used with conventional positive electrode systems (i.e., PbO.sub.2 /H.sub.2 SO.sub.4 /PbSO.sub.4), and thus does not suggest the uses contemplated by the present invention.
Further, the use of bulk titanium oxide having the formula TiO.sub.x where x is 1.55 to 1.95 has been suggested for electrode use in electrochemical cells. See, Hayfield, U.S. Pat. No. 4,422,917 issued Dec. 27, 1983. Only solid, bulk materials are discussed for electrode applications including storage batteries, electrochemical cells for chlorate production, etc.
A pair of co-pending, commonly assigned applications, Ser. No. 07/345,993 filed May 2, 1989, now U.S. Pat. No. 5,045,170, to Bullock et al. or 07/426,580 filed Oct. 24, 1989, now U.S. Pat. No. 5,017,446, to Reichman, et al. describe the use of certain conductive oxides as electrode materials and suggest the use thereof in a plastic binder as a bipolar substrate additive. Neither application suggests the use of such a substrate material along with an adjoining layer of carbon-filled plastic
The present invention provides a novel substrate structure for use in bipolar batteries which is not appreciated by the foregoing art and which overcome the deficiencies of the aforementioned systems.